1) Field of the Invention
The present invention relates to a packaging resin useful in the food industry based on polyethylene terephthalate (PET) polymer. In particular, the present invention is a copolymer based on an oxygen scavenging compound and a PET based polymer containing from about 10 to about 120 parts per million (ppm) cobalt (Co) based on the PET polymer, and from about 15 to 150 ppm zinc (Zn) based on the PET polymer. Both Co and Zn are catalyst for the oxygen scavenging compound, and additionally may serve as catalyst for the ester interchange reaction, if the PET based polymer is produced using such a reaction process. This copolymer may be used to produce packaging films, beverage containers such as soft drinks or beer bottles, or food containers such as ketchup bottles, baby food containers, etc.
2) Prior Art
Oxygen scavenging polymers are well known and are especially useful in the food packaging business. It is known that oxygen can have an effect on the odor and taste of packaged food thereby shortening the shelf life of the food. Oxygen scavenging packaging materials, on the other hand, react with oxygen that is in the process of traversing the packaging barrier. Thus the oxygen scavenging packaging material reduces or eliminates the odor and/or undesirable taste of food or beverages excessively exposed to oxygen.
It is known to use ethylene acrylate copolymers, which are modified with selected cyclic and allylic pendent groups, in oxygen scavenging packaging materials. It is also known to use polyethylene terephthalate or polyethylene naphthalate in conjunction with an oxygen scavenging polymer. The most frequently disclosed oxygen scavenging material is polybutadiene and copolymers thereof having cyclic and allylic pendant groups. However, oxygen scavenging materials, such as polybutadiene for example, need a catalyst to effectively react with oxygen. Well known in the art as catalysts for oxygen scavenging materials are the transition metal catalysts. Compounds, such as salts of transition metal elements are typical catalysts.
PCT Patent Application WO 98/12244 in the name of inventors Cahill et al. for Amoco Corporation discloses an oxygen scavenging composition wherein the critical oxygen scavenging material is polybutadiene and the catalyst for the oxygen scavenging material is transition metal salts or other compounds. This application discloses the usefulness of such a composition as a blend with polymers for packaging films and containers for the food and beverage industry.
PCT Patent Application WO 99/48963 in the name of Ching et al. for Chevron Chemical Company discloses an oxygen scavenging composition including a polymer or oligomer having at least one cyclohexene group or functionality. This application also discloses the use of transition elements as a catalyst to activate the oxygen scavenging composition. The transition metal catalysts are employed in the form of salts and other compositions. This reference also notes that cobalt, a transition metal catalyst, is preferred.
It is known to use a cobalt compound as an esterification catalyst for producing PET when the starting raw materials are esters of dicarboxylic acids and glycols, for example. Cobalt is also typically used to balance the color properties of PET so that it has a more neutral clear color. Patents teaching cobalt compounds"" use as an ester interchange catalyst or as an additive to balance the color are numerous.
U.S. Patent Re. 32,765 to Callander et al teaches high clarity colorless polyesters. To achieve this, a cobalt compound is used which neutralizes the yellow color. This patent also cautions against the over use of cobalt because it increases the acetaldehyde level in a container. Acetaldehyde destroys the taste of cola soft drinks, for example.
U.S. Pat. No. 5,008,230 to Nichols teaches a catalyst system for producing PET using both zinc and cobalt compounds. In this system, the catalysts are used to balance the color properties and reduce the polymerization time of PET made from terephthalic acid and ethylene glycol.
Oxygen scavenging compositions using transition metal catalysts, and particularly cobalt compounds, are effective in packaging polymer applications, nevertheless, their performance needs improvement. Improvement in the performance permits longer shelf life for the food. Additionally, when too much cobalt catalyst is employed in oxygen scavenging compositions, the color of the polymer over corrects, becoming too blue, and is unacceptable. Therefore, there is a need to balance the color of the polymer with an improved oxygen scavenging ability.
The present invention improves the effectiveness of the oxygen scavenging composition and maintains an excellent balance in the color properties of the PET based polymer. It is known that too much cobalt provides too much blue coloring, and can sometimes also produce haziness. On the other hand, some cobalt is generally desirable when forming PET based polymer to balance the yellow coloring, which generally appears during the production thereof.
The present invention has a synergistic effect in the use of zinc as a catalyst for the oxygen scavenging material compared with other transition metal catalysts. This has not been recognized prior to the present invention.
In the broadest sense, the present invention relates to an improved oxygen scavenging PET based copolymer comprising from about 10 to about 120 ppm cobalt based on the PET polymer, and from about 15 to about 150 ppm zinc based on the PET polymer.
In the broadest sense, the present invention also comprises: producing a polyester using Zn and Co as catalysts or coloring agents, copolymerizing an oxygen scavenging compound with the polyester, and extruding the resultant copolymer.
In the broadest sense, the present invention also comprises a process for preparing a PET based oxygen scavenging copolymer, comprising the steps of:
a) polymerizing a PET based polymer;
b) adding Zn, Co, and an oxygen scavenging compound during said polymerizing step;
c) copolymerizing said oxygen scavenging compound with said PET based polymer to form a copolymer; and
d) extruding said copolymer.
In the broadest sense, the present invention also comprises a process for preparing a PET based oxygen scavenging copolymer, comprising the steps of:
a) providing an extruder with a heated mixing zone;
b) feeding a PET based polymer, containing one or both Co and Zn catalyst, to said extruder;
c) feeding to said extruder, materials selected from the class consisting of:
i) an oxygen scavenging compound and Zn, if Zn is not in said PET;
ii) an oxygen scavenging compound and Co, if Co is not in said PET; and
iii) an oxygen scavenging compound if said PET contains both Co and Zn;
d) copolymerizing in said extruder said oxygen scavenging compound with said PET based polymer to form a copolymer; and
e) extruding said copolymer.
In the broadest sense, the present invention also comprises an oxygen barrier container having one or more layers of a PET based copolymer having from about 10 to about 120 ppm Co based on said PET polymer, and from about 15 to about 150 ppm Zn based on the PET polymer; and an oxygen scavenging compound wherein said Co and said Zn are catalysts for said oxygen scavenging compound.